Optical networks are widely used for data communication in backbones as well as for access networks. In such systems, detection of faulty operation conditions is of importance in order to avoid breakdowns of the network system or other disadvantages. For example in a PON network (passive optical networks), a point to multipoint network system is established between a common optical line termination device (OLT) and a plurality of optical network termination devices (ONT). As in PON networks communication is essential a broadcast communication where simultaneously transmission of optical signals on the network transmitted by more than one of the ONT devices to the OLT device is not allowed, a faulty operation of one of the ONT devices resulting in a permanently sending of optical power over the PON system totally blocks data communication to the OLT device for the other ONT devices.